Mobile radiotelephone with handsfree device

ABSTRACT

A mobile radio transceiver comprising a hands-free facility, which hands-free facility combines at least two acoustic input signals to produce an output signal, and an adaptive filter is provided for filtering the combined output signal. To achieve an improved speech quality, a high-pass filter is provided for filtering the acoustic input signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mobile radio transceiver with a hands-freefacility, which hands-free facility comprises combining means forcombining at least two acoustic input signals, and an adaptive filterfor filtering the combined output signal of the combining means.

The invention further relates to a hands-free facility which comprisescombining means for combining at least two acoustic input signals, andan adaptive filter for filtering the combined output signal of thecombining means.

2. Discussion of the Related Art

A hands-free facility with four microphones is known, for example, from"A MICROPHONE ARRAY WITH ADAPTIVE POSTFILTERING FOR NOISE REDUCTION INREVERBERANT ROOM" by Rainer Zelinski, ICASSP 88, pp. 2578 to 2581. In afirst processing step the directivity gain of the two-dimensionallyarranged microphones is used for noise reduction. In a second processingstep the microphone signals are further processed by an adaptive Wienerfilter which estimates the desired speech signal. For computing theparameters of the Wiener filter the autocorrelation function and thecrosscorrelation function of the input signals are measured.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the speech quality in amobile radio transceiver and in a hands-free facility of the typedefined in the opening paragraph.

In a mobile radio transceiver as well as in a hands-free facility of thetype defined in the opening paragraph this object is achieved in that ahigh-pass filter is provided for filtering the acoustic input signals.

The invention is based on the recognition that various undesired sideeffects occur as a result of the large noise component in the microphonesignals i.e., in the acoustic input signals in a motor car when theparameters for the adaptive filter are computed, which side effects leadto a degradation of the speech quality of the output signal. Forprocessing the acoustic analog input signals coming from a group ofmicrophones arranged in an array, these input signals are firstconverted, for example, into digital signals by analog-to-digitalconverters. The combining means following in the circuit are used forcombining the digital input signals thus obtained to an output signalthat contains the least possible interference. The adaptive filterarranged downstream of the combining means is used for reducingdisturbing noise which often occurs with a hands-free facility arrangedin a motor car, for example, as a result of engine noise. Especially theuse of high-grade microphones may call forth a degradation of the outputsignal of the hands-free facility.

In accordance with a principle of the invention the high-pass filteringof the acoustic input signals surprisingly simply achieves a reductionof these effects. For example, a 4^(th) order digital infinite impulseresponse (IIR) filter can be used as a high-pass filter. Subjecting theinput signals to such a high-pass filtering provides that the poorersignal processing relating to the low-frequency portions of the inputsignals in the hands-free facility may be omitted, so that on the wholean improvement of the speech quality of the output signal of thehands-free facility of the mobile radio transceiver is obtained. Thisachieves that the relatively high signal energy of the low-frequencyrange, which energy is mainly formed by interference signals, isfiltered out and thus an improvement of the speech quality of thehands-free facility and thus of the mobile radio transceiver iseffected.

An improved speech quality of the hands-free facility of the mobileradio transceiver may be achieved when the limit frequency of thehigh-pass filter lies in the range from 200to 400 Hz. Especially with a300 Hz limit frequency of the high-pass filter a considerableimprovement of the output signal could be realised, as frequencies lyingbelow 300 Hz do not play a role anyway in telephone speech relating to amobile radio transceiver.

A further improvement of the speech quality of the hands-free facilitymay be ensured in that a further high-pass filter is provided forfiltering the output signal of the adaptive filter, in accordance withan aspect of the invention. The further high-pass filtering may reducedfurther low-frequency interference signal portions that have not beensufficiently suppressed by the adaptive filter. This is based on therecognition that as a result of the time variance of the adaptivefilter, additional phenomena occur of which the energy is concentratedon the lower frequency range even if the input signals do not containlow-frequency signal portions. The further high-pass filteringattenuates these low-frequency signal portions and simultaneouslyincreases the higher signal frequencies, so that a dull sound impressionotherwise caused by a faulty reaction to an attenuation of thehigh-frequency signal portions is avoided.

A further improved speech quality of the hands-free facility of themobile radio transceiver may be achieved when the limit frequency of thefurther high-pass filter lies in the range from 200 to 400 Hz. Aconsiderable improvement of the output signal could be achievedespecially with a 300 Hz limit frequency of the further high-passfilter.

Differences of delay of the acoustic input signals recorded by, forexample, a microphone array, may be simply taken into account in thatthe combining means comprise a delay equalization means for equalizingthe differences between the input signals. Therefore, the delayequalization means is inserted between the microphone and the inputsignal summation circuit and used for equalizing the delay differencesbased on the time offsets between the speech signal portions of theacoustic input signals i.e. of the microphone signals.

In accordance with an aspect of the invention, a further improvement ofthe speech quality may be achieved in by including the delayequalization means, plausibility examining means provided for examiningwhether a defined distance and/or direction from the starting point ofthe output signals to a speaker exceeds a predefined limit value. Duringsuch a plausibility examination there is examined whether a defineddistance to the speaker is exceeded by the predefined limit value whenthe speaker is focused at, while this limit value can be selected inaccordance with the inside measurements of the motor car. Additionally,the direction from which the input signals are recorded can also betaken into account for the plausibility examination as a result of whicherroneous, i.e., impossible speaker locations, for example a speakerlocation behind the microphones, can be excluded.

A particularly poor speech quality occurring as a result of erroneouslyestimated correlation functions and/or numerical problems can beimproved in that the hands-free facility comprises an arrangement forcomputing the parameters of the adaptive filter from the autocorrelationfunction and the cross-correlation function of the input signals, whileespecially the autocorrelation function and/or cross-correlationfunction are averaged with time.

An advantageous possibility for computing the cross-correlation functionis that the autocorrelation function is scaled and the cross-correlationfunction is affected by the autocorrelation function.

Annoying effects during hands-free operation may simply be furtherreduced in that the hands-free facility comprises switching meansprovided for not using the adaptive filter when there is a coefficientoverflow during the computation of the autocorrelation function and/orthe cross-correlation function.

A Wiener filter may be suitably used as the adaptive filter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be further explained with referenceto the exemplary embodiments shown in the drawing Figures in which:

FIG. 1 shows a cutaway view of a motor car with a diagrammaticallyrepresented hands-free facility, and

FIG. 2 shows an embodiment for a hands-free facility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in a diagram a cutaway view of a motor car 7 with a driver3. FIG. 1 further shows that a mobile telephone 4 with a hands-freefacility 6 is arranged in the motor car, while the hands-free facility 6radiates a signal received from a remote subscriber via a loudspeaker 2into the acoustic environment i.e. into the inside of the motor car 7and is thus audible to the local subscriber 3 ("loud hearingoperation"). A microphone group 1 is arranged over the speaker 3 in themotor car 7, which microphone group captures the speech of the speaker 3as well as further acoustic signals from the environment to send them asacoustic input signals 9 via the hands-free facility 6 and the mobileradio transceiver 4 to a remote subscriber connected to the mobile radiotransceiver 4 by a radio channel 5 ("hands-free operation"), while inthe following "hands-free operation" is always intended to be understoodas the combination of the two functions of "loud hearing operation" and"hands-free operation". The hands-free facility 6 arranged inside themotor car may form both a constituent part of the mobile radiotransceiver 4 and be connected to the mobile radio transceiver 4 as aseparate arrangement. The further components normally used for theoperation of a mobile radio transceiver are not shown in the exemplaryembodiment shown in FIG. 1 for clarity.

During hands-free operation in a motor car, fundamental problems occur.On the one hand, the microphone 1 or the microphone group 1respectively, captures not only the desired speech signal of the speaker3 but also other signals from the acoustic environment, which othersignals can be classified into two categories with respect to theavailable technical solutions. They are, on the one hand, the acousticecho of the loudspeaker signal and, on the other hand, the local noisewhich may be caused by, for example, engine, ventilation, tyres, wind,rain and so on. These signals act as a disturbance on the actual usefulsignal i.e. the voice of the local subscriber 3 and may thus decisivelyaffect the speech quality of the telephone link established by themobile radio transceiver 4 and the radio channel 5. They degrade theunderstandability of the useful signal and impair the quality thereofalso when a speech coding method is used. The impairing effect alsoenhances when the transmission bit rate is reduced, for example, withGSM codecs and the recognition rate of voice recognizers which, forexample, can be provided for voice-controlled operation of the mobiletelephone 4 is reduced. For the actual telephone operation realised viathe mobile radio transceiver 4, the two former effects are important,whereas only the remote subscriber will be conscious thereof. Forexample, the remote subscriber will hear the echo of his own voice(after about 30 ms signal delay i.e. with GSM with about 180 ms delayall the more distinctly), and he will perceive the effects of localnoise as impaired speech quality. The local subscriber 3 will experiencereactions only if the feedback of the loudspeaker signal to themicrophone 1 causes stability problems to occur in the telephoneconnection. With respect to the voice recognition functions, forexample, when the mobile telephone 4 is voice controlled, the localsubscriber will immediately sense the effects of the disturbances by thedegradation of the recognition rates.

In addition to the desired loudspeaker signal 2 and the associatedechoes, the local subscriber 3 naturally additionally hears the localnoise, so that the sound impression and the understandability areaccordingly impaired when there is a high additional noise level. Toremove the local noise from the microphone signals 1, various conceptsfor noise reduction can be used. In principle, one always tries toanalyze the useful signal and the noise signal separately and tosuppress the noise signal in response to the properties found, withoutsimultaneously affecting the useful signal.

FIG. 2 shows an exemplary embodiment for a hands-free facility 6 as itcan be used in combination with or inside a mobile radio transceiver 4shown in FIG. 1 (cf. FIG. 1). The hands-free facility 6 shown in FIG. 2comprises a microphone group 1 arranged, for example, as a microphonearray, whose acoustic input signals 9 captured by the individualmicrophones are applied via an analog-to-digital converter 22 to ahigh-pass filter 11 to be filtered. The high-pass filter 11 forms partof the combining means 30 which are arranged for combining the inputsignals 9, while the combining means 30 further include an arrangement12 for optimum weighting of the input signals, a delay equalizationcircuit 13, a delay estimator 17 as well as a summation circuit 15. Acombined input signal 23 available on the output of the combining means30 is fed to an adaptive filter 14 which, together with the filterestimator 19, forms the filter means 40 of the hands-free facility 6.For filtering the output signal 24 of the adaptive filter 14 there isadditionally provided a further high-pass filter 21 which produces theoutput signal 22 of the hands-free facility 6. In the exemplaryembodiment for a hands-free facility 6 shown in FIG. 2 only the parts ofthe hands-free facility relevant to the scope of the invention areshown, while a representation of the further components of a customaryhands-free facility has been omitted for clarity.

The acoustic input signals 9 captured by the microphone group 1 arefirst converted into digital signals by converting means 22 (A/Dconverters). The subsequent combining means 30 are used for convertingthe digital input signals thus obtained into a minimum-noise outputsignal 23 whose quality is to be further improved by the subsequentfilter means 40. For this purpose, the digitized input signals 9 arefirst high-pass filtered by high-pass filter 11 in the hands-freefacility 6 shown in FIG. 2, so that in a surprisingly simple mannerinterference in the acoustic input signals 9 can be reduced. Such ahigh-pass filter may be arranged, for example, as a 2^(nd) to 4^(th)order digital IIR (Ifinite Impulse Response) filter. For example, 300 Hzis used as a limit frequency of the high-pass filter 11. Frequenciesbelow 300 Hz do not play any role for the speech signals to be processedfor a mobile radio transceiver and the telephone speech resultingtherefrom. Arrangement 12 performs an optimum weighting of the high-passfiltered input signals in a known manner, while the high-pass filteredinput signals on the output of the high-pass filter 11 are also used forestimating a delay for the delay estimator 16 as well as for the filterestimator 19 for estimating the parameters 20 of the adaptive filter 14.

Before the input signals are combined by the summation circuit 15, thedelays of the input signals 9 of the microphone group 1 are equalized bythe delay equalization circuit 13 in the exemplary embodiment for thehands-free facility 6 shown in FIG. 2. For this purpose the delayequalization circuit 13 comprises plausibility examining means providedfor examining whether a defined distance from the starting point of theinput signals 9 to a speaker exceeds a predeterminable limit value. Sucha plausibility examining circuit therefore investigates whether thedefined distance to a speaker exceeds a predeterminable limit value whenthe speaker is focused at. This limit value can be selected inaccordance with the inside dimensions of the motor car. For example,distances over one meter would mean that the speaker must be outside thevehicle. Additionally, the direction of the input signals can be takeninto consideration for such a plausibility examination. As a result,supposed speaker positions, for example, behind the microphones, thus inthe direction of the windscreen, may be recognized as unlikely as may bespeaker positions to the left of the driver's position. When an unlikelyspeaker position is recognized, preferably the most recent plausiblespeaker position, i.e. the previous estimates, is used. This may beensured by the switching means 18 included in the combining means 30shown in FIG. 2. The microphone array 1 with its subsequent delayequalization makes it possible to separate useful and noise componentsin the input signals 9 by its spatial selectivity. Simultaneously, anadditional array gain is achieved when the useful signal source isfocused at and the individual microphone signals 9 are averaged, if thenoise components in the microphone signals are alternately uncorrelatedand are therefore partly cancelled during the summation.

It is an object of the subsequent adaptive filter 14 of the filter means40 to accentuate the useful portions of the combining signal 23 andadditionally suppress the remaining noise signals. For this purpose, theautocorrelation function and the cross-correlation function of the inputsignals 9 are computed as parameters for the adaptive filter 14arranged, for example, as a Wiener filter, via the filter estimator 19.Furthermore, there is examined whether a coefficient overflow hasoccurred during the computation of autocorrelation function and/orcross-correlation function and in that case the adaptive filter 14 isnot to be used. This may also be effected by the switching means 18. Thefurther high-pass filter 21 can filter out the low-frequency noisesignal components in the signal 24 available on the output of theadaptive filter 14, which components are insufficiently suppressed bythe adaptive filter 14, so that the output signal 22 of the hands-freefacility 6 has a further enhanced speech quality.

We claim:
 1. A mobile radio transceiver with a hands-free facility, saidhands-free facility comprising:means for combining at least two acousticinput signals and providing a combined output signal, said means forcombining comprising a delay equalization circuit for equalizing delaysbetween the input signals, said delay equalization circuit comprisingplausibility examining means for examining whether a defined distanceand/or direction from a starting point of the input signals to a speakerexceeds a predeterminable limit value; an adaptive filter for filteringthe combined output signal of said combining means; and means forhigh-pass filtering each of the acoustic input signals.
 2. The mobileradio transceiver as claimed in claim 1, further comprising anarrangement for computing parameter of said adaptive filter from anautocorrelation function and a cross-correlation function of the inputsignals, wherein the autocorrelation function and/or cross-correlationfunction are averaged with time.
 3. The mobile radio transceiver asclaimed in claim 2, further wherein for computing the cross-correlationfunction the autocorrelation function is scaled and thecross-correlation function is affected by the autocorrelation function.4. The mobile radio transceiver as claimed in claim 3, furthercomprising switching means for not using the adaptive filter when thereis a coefficient overflow during a computation of the autocorrelationfunction and/or the cross-correlation function.
 5. The mobile radiotransceiver as claimed in claim 4, wherein the adaptive filter is aWiener filter.
 6. A hands-free facility comprising:means for combiningat least two acoustic input signals and providing a combined outputsignal, said means for combining comprising a delay equalization circuitfor equalizing delays between the input signals, said delay equalizationcircuit comprising plausibility examining means for examining whether adefined distance and/or direction from a starting point of the inputsignals to a speaker exceeds a predeterminable limit value; an adaptivefilter for filtering the combined output signal of said combining means;a high-pass filter for filtering the acoustic input signals; and afurther high-pass filter for filtering an output signal of said adaptivefilter, the limit frequency of the further high-pass filter lying in therange from 200to 400 Hz.
 7. The hands-free facility as claimed in claim6, further comprising an arrangement for computing parameters of saidadaptive filter from an autocorrelation function and a cross-correlationfunction of the input signals, wherein the autocorrelation functionand/or crosscorrelation function are averaged with time.
 8. Thehands-free facility as claimed in claim 7, further wherein for computingthe cross-correlation function the autocorrelation function is scaledand the crosscorrelation function is affected by the autocorrelationfunction.
 9. The hands-free facility as claimed in claim 8, furthercomprising switching means for not using the adaptive filter when thereis a coefficient overflow during a computation of the autocorrelationfunction and/or the cross-correlation function.
 10. The hands-freefacility as claimed in claim 9, wherein the adaptive filter is a Wienerfilter.